arXiv subjects

Dea Daniella Wangsawijaya

Publications and source records attributed to Dea Daniella Wangsawijaya.

4 recordsLinked to original sources

arXiv2026

Effects of spatially localised pressure gradient histories on recovery of turbulent boundary layers

Hot-wire anemometry is used to investigate the recovery of smooth-wall turbulent boundary layers from spatially localised (i.e. impulsive) pressure gradient history (PGH) effects. Measurements are performed at multiple stations downstream of spatial distributions of favourable-adverse pressure gradient sequences, followed by relaxation to zero-pressure-gradient (ZPG) conditions. The analysis focuses on matched friction Reynolds numbers at $Re_τ\approx 2300$, $3000$, and $5500$, where the local Clauser pressure gradient (PG) parameter $β$ is nominally matched at $1.7$, $0.6$, and $-0.1$, respectively. PGH strength is quantified using the integral history parameter $Δβ$, proposed by Preskett et al. (J. Fluid Mech., vol. 1010, 2025, A30), which allows isolation of PGH as the primary source of variation. The imposed PGH amplifies the wake component of the mean velocity profile and enhances the streamwise Reynolds stress throughout the boundary layer, including the emergence of an outer peak. Spectral analysis reveals an additional outer-layer energetic feature with streamwise length scales of $2$-$3δ$ ($δ$ is the local boundary layer thickness), identified as the PG peak, distinguishable from the very-large-scale motion (VLSM). Even after $β$ has relaxed (to zero) for sufficiently long distances, mean flow has not recovered to ZPG state. Once $Δβ\lesssim 0.1$, mean flow and inner/log-layer turbulence statistics recover; however, the outer-layer turbulence retains a persistent imprint of PGH. Finally, we observe that recovery involves reorganisation of large-scale structures - where VLSMs appear to be shorter even after the PG peak has vanished - which indicates prolonged history effects.

physics.flu-dyn↗

arXiv2026

Characterisation of rough-wall drag in compressible turbulent boundary layers

In compressible turbulent boundary layers (TBLs), roughness drag is typically characterised by first applying a velocity transformation to account for compressibility, after which the momentum deficit $ΔU^+$ (Hama, 1954) and the equivalent sand-grain roughness $k_s$ are inferred. In practice, $k_s$ is often obtained from measurements at a single Mach number $M$ and Reynolds number $Re$, effectively forcing the roughness into the $ΔU^+$--$\log(k_s)$ relation of Nikuradse (1933). This raises a key question: if a rough surface has a known $k_s$ in incompressible flow, under what conditions can this value be used in compressible flows? This question is explored using data obtained through a series of experiments of TBLs on rough walls (P60- and P24-grit sandpapers) over $0.3 \leq M \leq 2.9$ and $7427 \leq Re_τ \leq 30292$, including independent variation of $Re_τ$ at $M=2$. Results show that $ΔU^+$ is largely insensitive to the velocity transformation, but the fully rough regime exhibits a Mach-number-dependent shift in the logarithmic relation. Three empirical scalings are examined: an equivalent incompressible $k_s$, a viscosity-scaled roughness $k_{*} = k/ν_\infty^+$ with $ν_\infty^+ = ν_\infty/ν_w$, and a correction factor $\sqrt{1/F_c}$ where $F_c$ depends on $T_\infty/T_w$. The last provides the most consistent improvement across datasets, although all corrections remain empirical and rely on smooth-wall compressibility transformations. This paves the way for future work to develop custom transformation for a rough-wall TBL that can account for roughness properties and other parameters including wall conditions.

physics.flu-dyn↗

arXiv2021

Time-averaged velocity and scalar fields of the flow surrounding a group of cylinders

We examine the characteristics of mean flow and scalar concentration characteristics of a turbulent boundary layer flow impinging on a cluster of tall obstacles (which can also be interpreted as a porous obstruction). The cluster is created with a group of cylinders of diameter $d$ and height $h$ arranged in a circular patch of diameter $D$.The solidity of the patch/obstruction is defined by $ϕ$ (the total planar area covered by cylinders), which is systematically varied ($0.098 \leq ϕ\leq 1$) by increasing the number of cylinders in a patch ($N_c$). A point source is placed at ground level upstream of the patch and its transport around the patch is examined. Time-averaged velocity and scalar fields, obtained from simultaneous planar PIV-PLIF (particle image velocimetry-planar laser-induced fluorescence) measurements, reveal that the characteristics of wake and flow above porous patches are heavily influenced by $ϕ$. In particular, we observe that the horizontal and vertical extent of the wake and scalar concentration downstream of the patches decreases and increases with $ϕ$, respectively. Here, the recirculation bubble is shifted closer to the trailing edge of the patches as $ϕ$ increases, limiting the flow from convecting downstream, decreasing the scalar concentration and virtually `extending' the patch in streamwise direction. As the bubble forms in the trailing edge, vertical bleeding increases and hence the concentration increases above patch where the cylinders appear to be `extend' vertically towards the freestream.

physics.flu-dyn↗

arXiv2021

Investigation of unsteady secondary flows and large-scale turbulence in heterogeneous turbulent boundary layers

Following the findings in \cite{wangsawijaya2020}, we re-examine the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various roughness wavelengths $0.32 \leq S/\overlineδ \leq 3.63$, where $S$ is the width of the roughness strips and $\overlineδ$ is the spanwise-averaged boundary-layer thickness. The heterogeneous cases induce counter-rotating secondary flows, and these are compared to the large-scale turbulent structures that occur naturally over the smooth wall. Both appear as meandering elongated high- and low-momentum streaks in the instantaneous flow field. Results suggest that the secondary flows might be spanwise-locked turbulent structures, with $S/\overlineδ$ governing the strength of the turbulent structures and possibly the efficacy of the surface in locking the structures in place (most effective when $S/\overlineδ \approx 1$). Conditional averages of the fluctuating velocity fields of both spanwise heterogeneous and smooth wall cases result in structures that are strongly reminiscent of the streak-vortex instability model. Secondary flows and large-scale structures coexist in the limits where either $S/\overlineδ \gg 1$ or $S/\overlineδ \ll 1$, where the secondary flows scale on $δ$ or $S$, respectively. When $S/\overlineδ \gg 1$, the secondary flows are locked about the roughness transition, while relatively unaltered large-scale structures occur further from the transition. In the case where $S/\overlineδ \ll 1$, $S$-scaled secondary flows are confined close to the surface, coexisting with unaltered larger scale turbulent structures that penetrate much deeper into the layer.

physics.flu-dyn↗